This study was aimed to investigate the association between mthfr gene polymorphisms and toxicity of HDMTX in acute lymphocytic leukemia patients. A total of 44 patients were selected, and DNA was extracted from their peripheral blood. PCR-RFLP was used to determine the genotypes of mthfr. The toxicity response of patients received HDMTX chemotherapy was observed. The results showed that the toxicity of HDMTX to carriers of the variant allele at codon 677 (CT or TT) increased, as compared with individuals with the common CC genotype (OR = 3.75, 95% CI 1 - 14, p = 0.04). In contrast, the toxicity of HDMTX to ALL patients with the variant allele at codon 1298 (AC or CC) decreased as compared with the common AA genotype carriers (OR = 0.12, 95% CI: 0.026 - 0.564, p = 0.007). As compared with carriers of the variant allele at coden 1298 (AC or CC), the toxicity of HDMTX to patients with TT genotype at 677 and AA genotype at 1298 increased (OR = 16.5, 95% CI: 0.026 - 0.564). It is concluded that mthfr gene polymorphisms associate with the toxicity of HDMTX chemotherapy in acute lymphocytic leukemia.